When a semiconductor is exposed to light, electrons are produced with a strong reduction action and positive holes with a strong oxidation action, breaking down electrons contacting the semiconductor by means of an oxidation-reduction action. This property of semiconductors is called photocatalysis, and since the discovery of the photolysis of water by semiconductor photoelectrodes (the so-called Honda-Fujishima effect), it has been widely studied as a useful technique of converting light to chemical energy. Efforts have also been made to apply this principle for example to 1) oxidation of organic compounds, 2) hydrogenation of unsaturated compounds and other forms of organic synthesis, 3) removal and degradation of harmful chemicals in waste water and exhaust gas, 4) sterilization, 5) degradation of surface dirt and the like.
Such semiconductors (photocatalysts) which have been already discovered include not only titanium dioxide (titania) but also vanadium pentoxide, zinc oxide, tungsten oxide, copper oxide, iron oxide, strontium titanate, barium titanate, sodium titanate, cadmium sulfate, zirconium dioxide, iron oxide and the like. These semiconductors are also known to be effective as photocatalysts when supporting a metal such as platinum, palladium, rhodium, ruthenium or the like as a co-catalyst.
Semiconductor powder has been frequently used in conventional photocatalyst studies, but in order for photocatalysts to be practical they need to be made into films. To this end, they have been fixed in resin or glass, or the semiconductor itself has been used in the form of a thin film. The problem is that there is not enough of the catalyst itself, and its effects are also not adequate. In order to increase the amount of catalyst it is sufficient to increase the area of the catalyst layer, but this is often difficult because of design limitations.
With semiconductors such as these it is possible to obtain electrode output by exposing an n-type semiconductor to light. Consequently, they are used in the electrodes and the like of wet photoelectric cells using the photosensitized degradation effect. In particular there has been much development of dye-sensitized solar cells in recent years. The principal structure of the semiconductor electrode, which is a working electrode, is composed of a dye sensitizer adsorbed by a porous semiconductor layer. Materials which are used for such semiconductors include titanium dioxide (titania), tin oxide, zinc oxide, niobium oxide and the like. Ruthenium complexes and the like are known as sensitizing dyes. These dye-sensitized solar cells promise to be simpler in structure and cheaper than conventional silicon solar cells, but the greatest obstacle to practical application is improving exchange efficiency.
Therefore, methods are being studied of increasing the specific surface area of the semiconductor with low density in order to obtain greater optical activity in a small volume in photocatalysts and photoelectrodes. In other words, methods are being studied of making semiconductors more porous.
For example, a method has been presented of obtaining a porous titanium oxide thin-film photocatalyst with pores of a uniform size on the surface by first coating a substrate with titania sol and then heating and baking it (for example, Japanese Patent Publication No. 2636158).
Alternatively, a method has been presented of supporting or coating a photocatalyst on a porous silica body having pores 2 to 50 nm in diameter (for example, Japanese Unexamined Patent Publication No. H10-151355).
For example, an oxide semiconductor electrode has been developed (Japanese Unexamined Patent Publication No. 2001-76772) having a conductive substrate and a porous oxide semiconductor layer comprising hollow particles made of metal oxide formed on the aforementioned conductive substrate (Japanese Unexamined Patent Publication No. 2001-76772).